1. Field of the Invention
This invention relates to membrane electrode devices for electrodeposition coating, and more particularly to a membrane electrode device for electrodeposition coating, in which a tubular membrane support member is used for fixing a membrane for electrodeposition coating.
2. Description of the Prior Art
The electrodeposition coating is broadly divided into two catgories including one is a coating material of the anion type and the other using a coating material of the cation type. Since, in either of these electrodeposition coatings, uniformity and adhesion of the coating on an article to be coated are excellent and the degree of causing pollution is low, they have recently been widely applied to the automatic coating film treatment of motor vehicle bodies and so forth. These coatings for example, have shown themselves to be particularly suitable for use in the prime coating and/or coat finishing coating of metal materials.
Out of the coating materials used in these electrodeposition coatings, as the aforesaid coating material of the anion type, one of which is a carboxyle group adhered to a resin having a molecular weight (MW) of 2000 so as to be water-soluble. While, as the aforesaid coating material of cation type, one, of which is an amino group attached to a resin component of the coating material so as to be water-soluble, is used. On the other hand, even with these water-soluble coating materials, the degrees of ionization after being dissolved in the water are very low. For this, at present, in the case of the coating material of the anion type, an alkaline neutralizing agent such as triethylamine is mixed thereinto. While in the case of the coating material of the cation type, an acidic neutralizing agent such as acetic acid is mixed thereinto. Whereby, in both cases, neutralizing is effected, respectively, to thereby increase the degrees of ionization in the water.
The neutralizing agents are mixed to increasing the degrees of ionization in accordance with the properties of the resin components of the respective coating materials as described above. On the other hand, when the electrodeposition treatment of the articles to be coated advances to decrease the resin component in the solution, the coating material should be successively supplied from the outside. Accordingly, in the aforesaid solution, there is accumulated amine or acetic acid as the neutralizing agent. Whereby a phenomenon such as redissolving of the coated surface or occurrence of pin holes is generated, so that the efficiency of the electrodeposition coating is impaired to a considerable extent. To solve this, as described in Japanese Patent kokoku (Post-Exam. Publn.) No. 22231/1970 for example, such a so-called pH control is performed for increasing the efficiency that, one electrode is separated from the article to be coated, said article being the other electrode, and an aqueous solution by use of an ion-exchange membrane or the like. Further, and amine or acetic acid is osmotically extracted by use of the ion-exchange membrane or the like, thereby preventing the neutralizing agent from increasing in the aqueous solution.
On the other hand, the aqueous solution surrounding the article to be coated in an electrodeposition bath is constantly agitated to increase the efficiency of the electrodeposition, and water for discharging the neutralizing agent on the other electrode's side, which is separated by the flat plate-shaped ion-exchange membrane is continuously supplied from the outside in a very low value though. To accomplish this, high or low alternate pressures are largely and repeatedly applied to both service of the flat plate-shaped ion-exchange membrane at all times.
The aforesaid alternate pressures are generated, even when during a process in which an article to be coated is suspended from a line the article is transferred into and out of a bath, in addition to the time of agitating the aqueous solution for the electrodeposition.
As for changes in the water pressure, in the case of an ion-exchange membrane commonly used such as membrane having a height of about 1 m and a width of about 50 cm), if a change in water pressure of 0.5 kg/cm.sup.2 is generated, then, totally, it results in a change in water pressure of 2500 kg, whereby a tensile force of about 8.5 kg/cm is repeatedly applied to a portion, to which the membrane is mounted in a direction perpendicular to a center line (in this case, if a bulge is generated as inclined at about 10.degree. to the center line, then a tensile force of 48 kg/cm in a tangent line of the bulge due to [8.5/sin10.degree.]. For this, the ion-exchange membrane is constantly in a state where part or the whole thereof is flexed, bending and stretching are repeatedly applied thereto, so that such disadvantages often occur that a thin one cannot be used at all, while, even a thick one is damaged in a short period of time (actually, in two or three days). This fact leads to a case where the ion-exchange membrane should be periodically exchanged in a short period of time, whereby such a situation takes place that, in replacing works, preparation of a crane or the like is necessitated, and, in addition, a line of the electrodeposition coating itself should be stopped in operation.
Further, such phenomena occur that impurities permeated through the ion-exchange membrane and impurities in the water adhere to the peripheral surface of the electrode and polarization occurs. Further air bubbles adhere thereto due to the electrolysis of water, and these particles of polarization and air bubbles cannot be removed completely in the conventional trickling-down water feed method. Thus presenting the disadvantage of that the efficiency of electrodeposition is lowered with time. For this reason, in the electrodeposition coating by the membrane electrode method according to the conventional technique, the working efficiency is very low, thereby presenting the disadvantage of an increased cost.
On the other hand, with the purpose of obviating the above-described disadvantages, the inventor of the present invention has proposed a tubular membrane electrode device (Japanese Utility Model Application No. 082002/1982; U.S. patent application Ser. No. 499,818).
This invention related to one electrode provided in association with the article to be coated, which is the other electrode. More specifically, the invention is of such an arrangement "that a membrane is wound around the outer surface of a tubular membrane support member having water permeability and formed of an insulating material, a tubular electrode is provided on the inner surface's side of the membrane support member at a given interval therefrom, and a water way is provided for an electrolyte flowing from the above to the bottom of the inner diameter's side of the tubular electrode, and thereafter, flowing along the outer peripheral surface of the tubular electrode to outside".
However, the membrane electrode device in the conventional example, in which the membrane is wound around the membrane support member, there are some unsolved problems involving the membrane support member.
More specifically, in the above-described electrode device, a cylindrical membrane support member formed of insulating material, wherein a multiplicity of through-holes 50A are formed in as shown in FIG. 18 is commonly used, and the other, wherein a porous member 60 formed by a sintering process is utilized as a base material and finished. In this case, with the one, wherein the multiplicity of through-holes 50A are formed in the cylindrical member 50, in effecting the electrodeposition as shown in FIG. 14, a percentage of lines D of electric force directed from a positive electrode to a negative electrode being obstructed by the cylindrical member 50 for supporting the membrane is high, whereby an effective operating area of the membrane 51 is considerably decreased, thus presenting the disadvantage of that the efficiency of electrodeposition coating is low.
The other one, wherein the porous member formed by the sintering process is utilized, sludge 62 such as iron oxide tends to block spaces formed in the porous cylindrical member 60 for supporting the membrane, which member is complicated in construction as shown in FIG. 19, whereby the effective operating area of the membrane 51 is decreased with time, thereby presenting the disadvantage of that the efficiency of electrodeposition coating is deteriorated with time.